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Optimized Amplitude Modulated Multiband RF Pulse Design

机译:优化的调幅多频带射频脉冲设计

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摘要

Purpose: Multiband pulses are characterized by highly temporallymodulated waveforms. Rapid phase or frequency modulationcan be extremely demanding on the performance of radiofrequency(RF) pulse generation, which can lead to errors that canbe avoided if pulses are restricted to amplitude modulation(AM) only. In this work, three existingmultiband pulse design techniquesare modified to produce AMwaveforms.Theory and Methods: Multiband refocusing pulses weredesigned using phase-optimization, time-shifting, and rootflipping.Each technique was constrained to produce AM pulsesby exploiting conjugate symmetry in their respective frequencydomain representations. Pulses were designed using the AM andunconstrained techniques for a range of multiband factors (ie,number of simultaneously excited slices), time-bandwidth products,and slice separations. Performance was compared byexamining the resulting effective pulse durations. Phantom and invivo experiments were conducted for validation.Results: Acquired data confirmed that AM pulses can produceprecise results when unconstrained designs may produce artifacts.The average duration of AM pulses is longer than theunconstrained versions. Averaged across a range of parameters,the duration cost for AM pulses was 26, 38, and 20% for phaseoptimizing,time-shifting and root-flipping, respectively.Conclusions: Amplitude modulation multiband pulses can beproduced for a relatively small increase in pulse duration.
机译:目的:多频带脉冲的特征在于高度时间调制的波形。快速相位或频率调制可能对射频(RF)脉冲生成的性能有极高的要求,如果脉冲仅限于幅度调制(AM),这会导致可以避免的错误。在这项工作中,对现有的三种多频带脉冲设计技术进行了修改以产生AM波形。理论和方法:使用相位优化,时移和根翻转设计了多频带重聚焦脉冲,每种技术都被限制在各自的频域表示中利用共轭对称性来产生AM脉冲。 。脉冲是使用AM和无约束技术针对一系列多频带因子(即同时激发的切片的数量),时间带宽乘积和切片分离设计的。通过检查产生的有效脉冲持续时间来比较性能。结果:获得的数据证实,当不受约束的设计可能会产生伪像时,AM脉冲可以产生精确的结果。AM脉冲的平均持续时间比不受约束的版本更长。在一系列参数上取平均值,用于相位优化,时移和根翻转的AM脉冲的持续时间分别为26%,38%和20%。结论:可以产生幅度调制多频带脉冲,而脉冲持续时间的增加相对较小。

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